High frequency PIN diodes have been used for switching applications in the past. See, for example, U.S. Pat. Nos. 4,712,020 and 4,883,984. The former patent discloses an RF switch which employs a PIN diode as the switching element. A first bias circuit provides forward dc current through the diode to turn it on, to enable RF energy to be transmitted through the diode. A second bias circuit provides a reverse dc current through the diode to turn it off and prevent RF transmission through it. A control circuit selectably enables the bias circuits. The latter patent discloses a PIN diode switch with a series circuit comprising two oppositely-poled PIN diodes, with a control current supplied to the junction between the two diodes through the collector-emitter path of a junction transistor and through an inductor. The switch according to that patent operates without high reverse bias voltages.
Prior PIN diode switches suffer several limitations, some of which are inherent in the use of PIN diodes. High frequency PIN diodes require substantial forward bias current in order to reduce their ohmic losses at the low end of the frequency band. This bias current must be supplied to the PIN diodes via an RF blocking circuit in order to avoid RF leakage back into the bias and control circuitry. The low end of the frequency band is typically about 1 to 2 Mhz (corresponding to a time constant of about one microsecond), and since the RF blocking circuit must have a very high impedance (at least 1000 ohms), it is practically impossible to pass the required high bias current through it in less than 50 microseconds. Furthermore, attempting to speed up the bias current through the RF blocking circuit creates, in general, extreme voltage transients, since the RF blocking device is an inductor and the voltage across it is proportional to the change in current (V=L di/dt). These extreme voltage transients tend to "contaminate" the communication spectrum when using the switches in frequency hopping communication systems, and they also stress the electronic components.
RF blocking circuits for high power high frequency signals are difficult to build due to the extreme bandwidth requirements (i.e., 1 to 30 MHz, or a 30-to-1 bandwidth ratio). Moreover, when attempting to pass high dc currents through such blocking circuits, the ferrite material which is used inside these circuits saturates, causing sever heating and signal distortions. To compensate for saturation, the ferrite size has to be substantially increase, resulting in RF blocking circuits of substantial size and weight. High dc bias currents passing through the RF blocking circuit also suffer ohmic losses (I.sup.2 R losses, where R is the ohmic resistance of the blocking circuit). This reduces the efficiency of the switch.
In addition, in order to turn off a high frequency PIN diode switch, a high reverse dc voltage is applied across the diode. The level of this dc voltage typically far exceeds the amplitude of the high frequency signal being switched. When attempting to rapidly turn off the PIN diode via the RF blocking circuit, extreme current will be generated, as required to clear the PIN diode of stored charges. This extreme current will cause the generation of substantial voltage transients, as already described.
There is therefore a need for a PIN diode switch that overcomes these drawbacks of prior PIN diode switches.
The present invention is such a switch. In the present invention, the RF blocking circuit passes only low level bias currents, minimizing ohmic losses, ferrite saturation and voltage transients. Moreover, the dc bias currents through the inductors in the blocking circuit are maintained at a constant level (i.e., di/dt is zero). The bias currents are switched between the PIN diode switch element and a dummy load, allowing ultra high speed switching without voltage transients. At the same time, the dc current through the RF blocking circuit is always balanced. That is, a current component that is flowing in one direction is being counteracted by a current component with a similar magnitude flowing in the opposite direction. This arrangement prevents ferrite saturation and voltage transients. The RF blocking circuit acts as a transmission line both for the forward bias current and the high voltage turn-off signal, while at the same time blocking RF leakage from the switch element into the rest of the circuitry.